Here is another link for solutions of the equations leading to a big bang model. For example time is in the denominator in this solution for the density $\rho$, giving infinity at time $t=0$.
$$\rho(t)=\frac{1}{32\pi G t^2}$$

One sees that as t=0 is approached the matter density blows up. General Relativity tells us that in the region of high masses the physical laws we have studied and modeled in flat space change and may not hold in the form we know them (for example conservation of energy). The flat space approximation does not hold. Approaching zero time where the density tends to infinity is even worse, indeterminacy in masses means also indeterminacy in the laws governing space time.

is the breakdown that rho(0) tends to infinity or is indeterminate? would you mind explaining why which is the problem?
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user24341May 13 '13 at 4:13

Physical laws are not made to deal with infinities. I know that there exists many mathematical infinities but I have not seen them to be useful in mathematical physical models. Physical laws are already modified in the region of large masses. As density tends to infinity the distortions of spacetime can no longer be approximated by flat space, where our physical laws are determined so the physical laws become indeterminate too.
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anna vMay 13 '13 at 4:18

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There's also the issue that physics at the Planck scale is not fully understood
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kleingordonMay 13 '13 at 7:00